关于Docker for Mac :cached绑定挂载延迟及osxfs缓存逻辑的技术问询

Great question—this is a super common point of confusion with osxfs caching modes, so let's break this down clearly.

First, let's recap the official docs you referenced to set the stage:

For directories mounted in cached mode, the host’s filesystem view is authoritative; writes performed in the container are immediately visible on the host, but writes performed on the host may take some time to be visible within the container.
For directories mounted in delegated mode, the container’s filesystem view is authoritative; writes performed in the container may not be immediately visible on the host.

The key misinterpretation here is what "authoritative" actually means—it doesn't refer to real-time synchronization, but rather to the source of truth for final consistency.

Why host-side changes have delayed visibility in the container #

Docker for Mac uses osxfs to bridge the macOS host filesystem with the Linux VM that runs containers. This cross-system bridge has inherent performance overhead, so caching is used to speed up routine operations.

In cached mode:

• The container caches file data from the host to avoid repeated expensive cross-system reads. This makes container-side reads fast, but it means the container won't immediately pick up every host-side change.
• "Authoritative" here means that whenever the container does check for updates (either when its cache expires, or when it performs an explicit refresh like reopening a file), it will always pull the latest state from the host. If there's a conflict between the container's cache and the host's actual file, the host's version wins every time.
• Container-to-host writes are immediate because the container doesn't cache writes—any change the container makes gets flushed straight to the host (since the host is the source of truth, the container can't override it with cached data).

How this contrasts with delegated mode #

To make it clearer, compare with delegated:

• Here the container is authoritative, so the host caches container-side data. Container writes are fast (cached locally in the VM), but host-side visibility is delayed until the cache syncs. If there's a conflict, the container's version is the source of truth.

Quick summary #

• cached: Host = source of truth (final consistency), container reads use cache (host changes take time to show up in container), container writes go straight to host (immediate host visibility).
• delegated: Container = source of truth (final consistency), host reads use cache (container changes take time to show up in host), container writes are cached locally (fast container performance).

内容的提问来源于stack exchange，提问作者Erty Seidohl